This proposal requests NIH support for the Third Biennial Meeting of the International Photodynamic Association, to be held in Buffalo, NY, July 18- 21, 1990. The topic will be the selective photodynamic destruction (photodynamic therapy) of abnormal tissue elements, both benign and neoplastic. The term "photodynamic therapy" (PDT) refers to the use of photosensitizing compounds, which have been found to accumulate in, among others, neoplastic cells and tumors, atheromatous plaques, as well as bacterial and viral components. Once sensitizers have localized in the target tissue, they can be activated by light of sensitizer-specific wavelengths to generate cytotoxic molecular species, and cause irreversible damage to the biological material. While the first decade of PDT research has provided information concerning the understanding of the basic mechanisms of action, as well as the safety and efficacy of its clinical application using mainly porphyrins as sensitizers, the past several years have been characterized by a surge in the development of second generation photosensitizers and new light sources. At the same time, multi-center Phase III clinical trials are ongoing, which will hopefully lead to the regulatory approval of PDT in the U.S. and abroad. Although regional and national PDT meetings are being held regularly, intermittent international gatherings are necessary to provide updates of pre-clinical and clinical results, as well as technical developments. To facilitate the international dissemination of information, the International Photodynamic Association was founded in 1986 and has since organized two successful meetings (Tokyo, 1986; London, 1988). The proposed meeting, for which funding is requested, will feature invited experts and solicited papers in the fields of photobiology, physics, and pre-clinical and clinical PDT research. Emphasis will be placed on drug and light source development, as well as the current status of clinical trials and new clinical applications.